Metal-free arylations via photochemical activation of the Ar-OSO 2R bond in aryl nonaflates

Article abstract of DOI:10.1039/c3gc41009a

The photolysis of electron-rich aryl nonaflates (ArONfs) in protic media was investigated and heterolysis of the Ar-OS bond (from ³ArONf) took place. The reaction generated a triplet phenyl cation that added to π-bond nucleophiles. This metal-free arylation method was made further useful by adopting in situ preparation of ArONf from the corresponding phenol.

Full text of DOI:10.1039/c3gc41009a

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Metal-free arylations via photochemical activation of
the Ar–OSO2R bond in aryl nonaflates†
Cite this: Green Chem., 2013, 15, 2704
Received 29th May 2013,
Accepted 22nd July 2013
Carlotta Raviola, Veronica Canevari, Stefano Protti, Angelo Albini and
Maurizio Fagnoni*
DOI: 10.1039/c3gc41009a
www.rsc.org/greenchem
The photolysis of electron-rich aryl nonaflates (ArONfs) in protic exploring the photochemistry of ArONfs, not known as yet. In
media was investigated and heterolysis of the Ar–OS bond (from general, aryl sulfonates (I) are expected to undergo ArO–S
3ArONf) took place. The reaction generated a triplet phenyl cation bond cleavage via direct homolysis (path a, Scheme 1), but
that added to π-bond nucleophiles. This metal-free arylation electron-donating substituents made possible the heterolytic
method was made further useful by adopting in situ preparation cleavage of the Ar–OSO2R bond to give aryl cations trappable
of ArONf from the corresponding phenol.
by nucleophiles (path b). Apart from the synthetic application,
sulfonates I are of interest as neutral photoacid generators
since sulfonic acids are liberated.10,12,13 4-N,N-Dimethylamino-
(1a), 4-methoxy- (1b) and 4-t-butylphenyl nonaflates (1c) were
chosen as representative examples.
These absorbed in the UV at a wavelength depending on
the donating nature of the substituent, from 308 nm for 1a to
261 nm for 1c, similar to other sulfonates13 and fluoresced
weakly (most intensively 1a, λ = 377 nm, ΦF = 0.024, see
Table S1 in ESI†). They were shown to react efficiently under
irradiation (consumption quantum yield ranging from 0.39 to
0.78, no thermal reaction as shown by blank experiments). As
apparent from Fig. S1,† λ = 254 nm was required for the
irradiation of 1b,c whereas the more red-shifted spectrum of
1a allowed us to conveniently adopt 310 nm lamps. The pro-
ducts obtained in polar and protic solvents are indicated in
Table 1, classed as resulting from Ar–OS or ArO–S bond
Perfluoroalkane aryl sulfonates, such as triflates (ArO-
SO2CF3vArOTfs)1 and their C4 homologues nonaflates (ArO-
SO2C4F9vArONfs), are considered as appealing alternatives to
aryl halides in transition-metal mediated cross-coupling reac-
tions. This is due to the electron-withdrawing ability of the
perfluoroalkyl chain that favours the insertion of the metal
into the carbon–oxygen bond.2 ArONfs have recently found
widespread use in several cross-coupling procedures,3,4 includ-
ing the Pd catalysed reduction of aryl nonaflates to the corres-
ponding arenes,5 and are described as more reactive than aryl
mesylates and tosylates.6 Other advantages of ArONfs are their
stability to chromatography and storage at room temperature
and their smooth preparation from the corresponding phenols
by reaction with non-air-/moisture-sensitive nonafluorobutane-
sulfonyl fluoride, a reagent that is cheaper than trifluoro-
methanesulfonyl anhydride used in the preparation of aryl
triflates.7 Recently, an appealing one-pot approach to trans-
form the aryl–oxygen bond in phenols into valuable Ar–C or
Ar–F bonds has been devised by metal-mediated reactions
having recourse to in situ formed ArONfs.4b,6,8 In contrast, no
metal-free approaches are available so far for the activation of
the Ar–O bond in ArONfs. However, we demonstrated that a
metal-free route was accessible in the case of aryl mesylates
and triflates via photogenerated phenyl cations,9,10 intermedi-
ates not obtained by heating alkyl substituted ArONfs in tri-
fluoroacetic acid at 150 °C for 21 days.11 This suggested
PhotoGreen Lab, Department of Chemistry, University of Pavia, Viale Taramelli 12,
27100 Pavia, Italy. E-mail: fagnoni@unipv.it; Tel: +390382987316
†Electronic supplementary information (ESI) available: Experimental details,
copy of 1H and 13C NMR of 1a–1e, 6c, 7c, 8b,c, 10b, 10d, 10e and 11b. See DOI:
10.1039/c3gc41009a
Scheme 1 Competitive Ar–OS vs. ArO–S bond photocleavage in aryl
sulfonates.
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Table 1 Photocleavage of the ArO–S or Ar–OS bonds in the irradiation of aryl
nonaflates 1a (at 310 nm) and 1b–c (at 254 nm) in neat solvents
Products
ArONf
Entry (0.05 M)
Solventa
(Φ−1
tirr
)
(h) ArO–Sb (%)
Ar–OSb (%)
1
2
3
MeCN (0.63)
MeOH (0.78)
TFEc
4
3
4
2a, 45
—
—
—
2a, 98
2a, 28; 3a,d;
4a,d
4
5
6
MeOH (0.62)
MeCNe
5
2b, 45; 3b,d
5b, 1
5b, 7
5b, 12; 8b, 7
24 3b,d
MeCN–H2O
24 2b, 7; 3b,d
(5/1)e
7
8
9
10
11
12
MeOHe
24 2b, 10; 3b,d
24 3b,d
5
24 2c, 13
17 2c, 10
24
5b, 15; 6b, 10
5b, 11; 7b, 18
5c, 1; 6c, 7
5c, 4; 8c, 37
5c, 26; 6c, 33
5c, 11; 7c, 35;
9c, 8
Chart 1 Products arising from the photocleavage of the ArO–S (2–4) and
Ar–OS (5–9) bonds in aryl nonaflates 1a–c.
TFEc,e
MeOH (0.39)
2c, 65; 3c,d
MeCNe
MeOHe
π bond nucleophiles (mesitylene, benzene, 4-pentenoic acid,
allyltrimethylsilane (ATMS), 1-hexyne, see Table 2). These traps
did not affect the photochemistry of 1a and 2a was again the
only product formed. On the other hand, satisfactory arylation
was obtained from both 1b (in all of the cases tested) and 1c
(only with aromatic traps in TFE) at the expense of the pro-
cesses observed in neat solvent. Thus, biaryl 10b (79–86%
yields) was obtained in the presence of an excess of mesitylene
(70% when lowering the trap concentration to 0.2 M, entry 5,
TFEc,e
—
a Quantum yield of disappearance (Φ−1) measured at 254 nm. b Yields
were based on consumed 1a–c (>70% consumption observed in each
case) and determined by GC analysis. c Cs2CO3 0.03 M added; TFE =
2,2,2-trifluoroethanol. d Products detected in trace amounts by GC-MS
analysis. e Under 10% v/v acetone sensitization λ = 310 nm.
cleavage. The latter process was virtually exclusive with 1a Table 2) and 11b (up to 43% yield) with benzene in TFE. Estra-
(complete consumption after 3–4 hours irradiation) and gave gole 13b (57–61%) and a γ-lactone (12b, 62%) were obtained
4-N,N-dimethylaminophenol (2a). Only in 2,2,2-trifluoro- with allyltrimethylsilane and 4-pentenoic acid (0.5 M, TFE as
ethanol (TFE, entry 3), other products (the aryl nonafluorobu- the solvent), as well as alkyne 14b with 1-hexyne, though in a
tyl ether 3a and the nonafluorobutyl phenol 4a) were detected modest yield (37%).
in small amounts (<5% yield) by GC-MS analyses (Table 1).
Biaryls 10c and 11c (51 and 71% yield) were formed from
The same process predominated with 1b,c and led to 2b,c and 1c, but with 4-pentenoic acid the yields of the trapping
3b,c upon direct irradiation in MeOH at 254 nm, along with product, benzyl lactone 12c, and that of phenol 2c were com-
small amounts of products derived from Ar–OS bond cleavage parable (entry 13, Table 2) while no trapping occurred with
(5b,c and 6c, entries 4 and 9). Carrying out the same process ATMS. No arylation was observed when omitting acetone in
under acetone sensitization (10% v/v, irradiation at 310 nm, the experiments with 1b,c (in particular, irradiation of 1c in
3
24 hours), with the aim of generating 1a–c by energy transfer the presence of pentenoic acid either at 254 or at 310 nm led
from 3Me2CO, however, shifted the photoreaction toward to no arylation). Two further electron-rich aryl nonaflates were
Ar–OS cleavage, resulting in reduction (5b,c) and/or solvolysis tested in the presence of mesitylene. Thus, benzodioxole 1d
(6b–8b, 6c–8c) of the aryl sulfonate. In detail, anisole 5b was afforded 10d (49%, along with a small amount of sesamol 2d)
formed in 7–15% yield from 1b under all of the tested con- and the monononafluorobutanesulfonate of hydroquinone
ditions whereas the yield of solvolysis products (6b–8b) (1e, easily obtained from 1b by demethylation with BBr3) gave
increased with the proticity of the solvent reaching
a
biphenyl 10e in 80% yield in MeCN.
maximum value in TFE (entry 8). The ArO–S cleavage occurred
In the latter case, the effect of triethylamine (TEA) was
as a secondary pathway only in MeOH and MeCN–H2O yield- tested (entry 16). The amine prevented thermal deprotection of
ing phenol 2b along with traces of the photoextrusion product 1e and partially formed the phenate anion, thus increasing
(3b). Solvolysis was the main process also from the t-butyl- electron-donation toward the benzene ring. This led to no
phenyl nonaflate 1c and led to ethers 6c, 7c and amide 8c in a improvement, however (10e 62–67%) even in protic TFE as the
discrete yield (in TFE, 8% of 4-fluoro-t-butylbenzene 9c was reaction medium.
also formed). Unfortunately, in some cases the overall mass
balance was very poor probably due to polymerization in situ generation for the one-pot metal-catalyzed aminocarbo-
nylation and alkoxycarbonylation of phenols,6,8 or for the con-
The easy preparation of aryl nonaflates allowed for their
(Chart 1).
The above explorative results showed that heterolysis of the version of aryl alcohols into aryl fluorides.4b With the aim of
Ar–OS bond could be obtained under acetone sensitization. performing a related one-pot metal-free photoarylation of
We were thus encouraged to further explore the photoreactivity phenols we tested the in situ preparation of the aryl nonaflates
of a set of substituted aryl nonaflates (1a–e) in the presence of followed by irradiation in the presence of the chosen
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Table 2 Metal-free arylations by irradiation of nonaflates 1a–1e (0.05 M, at 310 nm) in the presence of π nucleophiles
Entry
ArONf (0.05 M)
Conditions
Ar–Nua (%)
Othersb (%)
2a, 32
1
2
3
1a
TFE, Cs2CO3 0.03 M,
Mesitylene (0.5 M)
TFE, Cs2CO3 0.03 M
4-Pentenoic acid (0.5 M)
MeCNc
—
—
2a, 40
1b
5b, 3
Mesitylene (0.5 M)
4
5
6
MeOHc
10b, 86b
10b, 70
5b, 5
Mesitylene (0.5 M)
TFE, Cs2CO3 0.03 Mc
Mesitylene (0.2 M)
TFE, Cs2CO3 0.03 Mc
Benzene (0.5 M)
2b, 4; 5b, 3
5b, 5
7
TFE Cs2CO3 0.03 Mc
4-Pentenoic acid (0.5 M)
5b, 14
8
MeCN–H2O (5/1)c
ATMS (0.5 M)
2b, 19; 5b, 7
9
TFE, Cs2CO3 0.03 Mc
ATMS (0.5 M)
13b, 57
2b, 9; 5b, 3; 7b, 6
5b, 3
10
TFE, Cs2CO3 0.03 Mc
1-Hexyne (0.5 M)
11
1c
TFE, Cs2CO3 0.03 Mc
Mesitylene (0.5 M)
7c, 5
12
13
TFE, Cs2CO3 0.03 Mc
Benzene (0.5 M)
5c, 2; 7c, 12; 9c, 1
2c, 30; 7c 16
TFE Cs2CO3 0.03 Mc
4-Pentenoic acid (0.5 M)
14
15
TFE Cs2CO3 0.03 Mc
ATMS (0.5 M)
—
2c, 12; 5c, 8; 7c,13
2d, 2
TFE, Cs2CO3 0.03 Mc
Mesitylene (0.5 M)
16
MeCNc
Mesitylene (0.5 M)
—
17
TFE, TEA 0.05 Mc
Mesitylene (0.5 M)
10e, 62
—
a Isolated yields based on consumed 1a–e. b Yields determined by gas chromatography (GC) analysis based on consumed ArONf. c Under 10% v/v acetone
sensitization. d Triethylamine (TEA) 0.05 M added. TFE = 2,2,2-trifluoroethanol.
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nucleophile. This required some work since TFE (in most
cases the elective solvent of the photoarylation) was unsuitable
for the preparation of aryl nonaflates and we thus adapted the
procedure reported by Beller and co-workers.6 The one-pot
photoarylation of 1c with mesitylene was chosen as a model
reaction and various solvents and bases were tested (see
details in ESI, Table S3†). Conversion of the phenol into the
sulfonate is optimal in neat MeCN, as already reported,6 and
insignificant in TFE, while the two solvents have an opposite
effect on the arylation.
A compromise was found and involved adding under stir-
ring Cs2CO3 and C4F9SO2F to a MeCN solution of 2c in a
quartz tube. After 15 min the reaction was complete and TFE
(to achieve a 0.05 M concentration of nonaflate), a further
amount of Cs2CO3 (0.03 M), acetone (10% v/v) and the chosen
π nucleophile were added. The resulting solution was nitrogen
purged and irradiated for 24 hours at 310 nm. The results are
gathered in Table 3. In one of the cases (synthesis of 10b) neat
MeCN was found to be more convenient than MeCN–TFE (1/9)
as the reaction solvent. Arylation yields were lower than, but in
some cases comparable to, those carried out by starting from
the purified aryl nonaflates. The procedure was further
extended to the corresponding nonaflates of 2- and 3-methoxy-
phenol (1f,g) by using mesitylene as a model nucleophile and
the corresponding biaryls 10f,g were obtained in a good yield
(65% and 57% respectively, entries 8 and 9). The results
obtained demonstrated that under suitable conditions the
Scheme 2 Different paths in the photoreactivity of aryl nonaflates 1a–g.
photochemical (heterolytic) activation of the Ar–OS bond for
metal-free arylations is indeed feasible for aryl nonaflates
despite (homolytic) competitive ArO–S cleavage from 11
(Scheme 2, path d). The latter path leads to a radical pair
(ArO˙, C4F9SO2˙) and hence to phenol 213 (Scheme 2, path f )
and indeed is the exclusive process with the amino derivative
1a. Perfluorinated radicals C4F9˙ are generated by release of
SO2 from perfluoroalkylsulfonyl radicals13–16 leading to ether 3
and perfluoroalkylated phenol 4 by coupling with Ar–O˙ (not
previously reported upon photolysis of aryl sulfonates).10
The synthetically useful point is the generation of the
3
triplet phenyl cation Ar+ (see Scheme 2). This is conveniently
arrived at by triplet sensitization.17 In fact, the presence of
acetone (ET = 80 kcal mol−1) gave direct access to 31 and a
Table 3 Arylations via in situ formed aryl nonaflates 1a
3
phenyl cation Ar+ (path b) of the same multiplicity from it.18
In neat solvent, hydrogen (or fluorine from TFE)19 abstraction
from the medium to form products 5,9 (path g) competes with
intersystem crossing to the singlet phenyl cation (1Ar+) that
ultimately affords solvolysis products 6–8 (path h).20
Othersa
(%)
However, when irradiation is carried out in the presence of
π bond nucleophiles, trapping of the triplet phenyl cation
becomes the main (or exclusive) path and arylation occurs
efficiently (products 10–14, paths a → b → c, see bold arrows)
along with the formation of variable (often negligible)
amounts of byproducts. The heterolytic cleavage of a C(sp2)–O
bond in nonaflates has been reported so far only in the
thermal solvolysis of vinyl nonaflates in protic solvents.21
This work demonstrates the viability of Ar–O bond hetero-
lysis in nonaflates and gives insight into the possible appli-
cation of these esters in organic synthesis, offering the first
application of aryl nonaflates in transition metal-free aryla-
tions. The arylation yields were in most cases lower than in the
related Pd-catalyzed reactions3 but here there is no need to
elaborate on the nucleophiles (e.g. simple aromatics could be
used in place of arylzinc derivatives or arylstannanes) or to
introduce auxiliaries (co-catalyst, or a base).
Entry ArONf
NuY (M)
Ar–Nub (%)
1
2
3
1b
Mesitylene (0.5 M) 10b, 51, 87c
Benzene (0.5 M)
4-Pentenoic acid
(0.5 M)
ATMS (0.5 M)
Mesitylene (0.5 M) 10c, 55
4-Pentenoic acid
(0.5 M)
Mesitylene (0.5 M) 10d, 33
2b, 4; 5b, 3
5b, 5; 7b, 9
5b, 19; 7b,
7
5b, 3; 7b, 7
—
2b, 16; 7b,
12
—
11b, 20
12b, 42
4
5
6
13b, 62
1c
12c, 24
7
8
1d
Mesitylene (0.5 M)
—
9
Mesitylene (0.5 M)
—
Furthermore, the in situ generation of nonaflates 1b–g pro-
vides a useful method to perform a one-pot two step sequence
for the metal-free smooth substitution of an alkyl or aryl group
for an OH group via nonaflate esters (actually the first gene-
ration of a phenyl cation starting directly from an electron-rich
a 1 (0.05 M); 24 hours irradiation at 310 nm in MeCN–TFE (1/9), yields
determined by gas chromatography (GC) analysis based on consumed
ArONf. b Isolated yield. c Reaction carried out in neat MeCN.
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phenol). More work is needed, however, to exploit the capa-
bility of aryl sulfonate esters in green arylations.
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Acknowledgements
S.P. acknowledges MIUR, Rome (FIRB-Futuro in Ricerca 2008
project RBFR08J78Q) for financial support. This work has
been supported by the Fondazione Cariplo (grant no. 2012-
0186). We are grateful to F. Corana and B. Mannucci (Centro
Grandi Strumenti, University of Pavia) for their precious help.
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